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Summary: Ctors | Methods | Inherited Methods

# DoubleAccumulator

`public class DoubleAccumulator`
`` `extends Number` `implements Serializable`

java.lang.Object
↳ | java.lang.Number
| ↳ | java.util.concurrent.atomic.DoubleAccumulator

One or more variables that together maintain a running `double` value updated using a supplied function. When updates (method `accumulate(double)`) are contended across threads, the set of variables may grow dynamically to reduce contention. Method `get()` (or, equivalently, `doubleValue()`) returns the current value across the variables maintaining updates.

This class is usually preferable to alternatives when multiple threads update a common value that is used for purposes such as summary statistics that are frequently updated but less frequently read.

The supplied accumulator function should be side-effect-free, since it may be re-applied when attempted updates fail due to contention among threads. The function is applied with the current value as its first argument, and the given update as the second argument. For example, to maintain a running maximum value, you could supply `Double::max` along with `Double.NEGATIVE_INFINITY` as the identity. The order of accumulation within or across threads is not guaranteed. Thus, this class may not be applicable if numerical stability is required, especially when combining values of substantially different orders of magnitude.

Class `DoubleAdder` provides analogs of the functionality of this class for the common special case of maintaining sums. The call `new DoubleAdder()` is equivalent to `new DoubleAccumulator((x, y) -> x + y, 0.0)`.

This class extends `Number`, but does not define methods such as `equals`, `hashCode` and `compareTo` because instances are expected to be mutated, and so are not useful as collection keys.

## Summary

### Public constructors
`DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, double identity)` Creates a new instance using the given accumulator function and identity element.

### Public methods
`void` | `accumulate(double x)` Updates with the given value.
`double` | `doubleValue()` Equivalent to `get()`.
`float` | `floatValue()` Returns the current value as a `float` after a narrowing primitive conversion.
`double` | `get()` Returns the current value.
`double` | `getThenReset()` Equivalent in effect to `get()` followed by `reset()`.
`int` | `intValue()` Returns the current value as an `int` after a narrowing primitive conversion.
`long` | `longValue()` Returns the current value as a `long` after a narrowing primitive conversion.
`void` | `reset()` Resets variables maintaining updates to the identity value.
`String` | `toString()` Returns the String representation of the current value.

### Inherited methods
From class `java.lang.Number`
`byte` | `byteValue()` Returns the value of the specified number as a `byte`, which may involve rounding or truncation.
`abstract double` | `doubleValue()` Returns the value of the specified number as a `double`, which may involve rounding.
`abstract float` | `floatValue()` Returns the value of the specified number as a `float`, which may involve rounding.
`abstract int` | `intValue()` Returns the value of the specified number as an `int`, which may involve rounding or truncation.
`abstract long` | `longValue()` Returns the value of the specified number as a `long`, which may involve rounding or truncation.
`short` | `shortValue()` Returns the value of the specified number as a `short`, which may involve rounding or truncation.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.

## Public constructors

### DoubleAccumulator

    public DoubleAccumulator (DoubleBinaryOperator accumulatorFunction,
                    double identity)

Creates a new instance using the given accumulator function and identity element.

Parameters
`accumulatorFunction` | `DoubleBinaryOperator`: a side-effect-free function of two arguments
`identity` | `double`: identity (initial value) for the accumulator function

## Public methods

### accumulate

    public void accumulate (double x)

Updates with the given value.

Parameters
`x` | `double`: the value

### doubleValue

    public double doubleValue ()

Equivalent to `get()`.

Returns
`double` | the current value

### floatValue

    public float floatValue ()

Returns the current value as a `float` after a narrowing primitive conversion.

Returns
`float` | the numeric value represented by this object after conversion to type `float`.

### get

    public double get ()

Returns the current value. The returned value is NOT an atomic snapshot; invocation in the absence of concurrent updates returns an accurate result, but concurrent updates that occur while the value is being calculated might not be incorporated.

Returns
`double` | the current value

### getThenReset

    public double getThenReset ()

Equivalent in effect to `get()` followed by `reset()`. This method may apply for example during quiescent points between multithreaded computations. If there are updates concurrent with this method, the returned value is not guaranteed to be the final value occurring before the reset.

Returns
`double` | the value before reset

### intValue

    public int intValue ()

Returns the current value as an `int` after a narrowing primitive conversion.

Returns
`int` | the numeric value represented by this object after conversion to type `int`.

### longValue

    public long longValue ()

Returns the current value as a `long` after a narrowing primitive conversion.

Returns
`long` | the numeric value represented by this object after conversion to type `long`.

### reset

    public void reset ()

Resets variables maintaining updates to the identity value. This method may be a useful alternative to creating a new updater, but is only effective if there are no concurrent updates. Because this method is intrinsically racy, it should only be used when it is known that no threads are concurrently updating.

### toString

    public String toString ()

Returns the String representation of the current value.

Returns
`String` | the String representation of the current value

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